No, to the best of our knowledge you cannot take the road and vision test in Ontario when you are 15.

The information we have received from the Ministry of Transportation (MTO) for Ontario, Canada states that to apply for a license, you must be at least 16 years old and pass a vision test and a test of your knowledge of the rules of the road and traffic signs.

It would thus appear you cannot apply or take tests for a G1 until you turn 16 years of age. To get specific information on how to apply and receive your G1 in Ontario and start your driver's licensing process contact the MTO.